The objective of this study was to evaluate the effect of direct-fed-microbial addition to a TMR on lactation perfor-mance by midlactation dairy cows. A total of 112 Holstein cows (28 primipa-rous and 84 multiparous; 139 ± 47 DIM at trial initiation) were stratified by par-ity and DIM and randomly assigned to 14 pens of 8 cows each. Pens were then randomly assigned to 1 of 2 treatments in a continuous-lactation trial consisting of a 2-wk covariate adjustment period with cows fed the basal TMR followed by a 10-wk treatment period with cows fed their assigned treatment diets. The 2 treatments were basal TMR plus either 1 g/cow per day of a direct-fed micro-bial (BOV, 1 × 10 9 cfu/g Lactobacillus acidophilus NP51 and 2 × 10 9 cfu/g Pro-pionibacterium freudenreichii NP24) or placebo (control). Milk yield was similar between treatments and averaged 44.9 kg/d. There was a trend (P < 0.08) for DMI to be decreased by 0.4 kg/d per cow for BOV overall, and DMI was lower (P < 0.05) for BOV than control during wk 2, 3, 6, 9, and 10. However, measures of feed conversion (milk or component-corrected milk yields per unit DMI) were unaffected (P > 0.10) by treatment. Milk fat, protein and urea nitrogen concen-trations were unaffected (P > 0.10) by treatment and averaged 3.75%, 3.11%, and 15.2 mg/dL, respectively. Likewise, BW change and condition score did not differ (P > 0.10). Under the conditions of this study, with midlactation cows fed a 54% forage TMR (DM basis), there was no improvement in lactation perfor-mance from the inclusion of a direct-fed microbial in the diet.